1. Field of Technology
The present invention generally relates to a sealed lead storage battery and, more particularly, to an improvement in a safety valve used in the sealed lead storage battery.
2. Description of the Prior Art
A sealed lead storage battery is widely used in portable electric appliances as a source of electricity because, in view of the fact that a liquid electrolyte is soaked in a woven mat of glass fibers which serves as a separator and no electrolyte exists substantially in a free flow state, there is no substantial possibility that the liquid electrolyte may leak out of the battery casing. It is usual that the recharging of the sealed lead storage battery with electric current inevitably results in the generation of a gaseous body of oxygen from one or more positive electrode plates, and the oxygen gas so generated is removed by absorption into a corresponding number of negative electrode plates with no possibility of the gas leaking outside the battery casing.
However, when the sealed lead storage battery is recharged with electric current of a value higher than the rated current, the rate of generation of oxygen tends to be higher than the gas absorbing speed exhibited by the negative electrodes with the result that the internal pressure inside the battery casing increases. In view of this, it is a common practice to employ at least one safety valve in the battery casing to permit the excess gas inside the battery casing to be relieved to the atmosphere before the battery casing ruptures as a result of the increased internal pressure. The safety valve generally used in the prior art sealed lead storage battery is generally required to have a dual function; as a pressure relief valve and as a flow check valve. While the safety valve works as a pressure relief valve when the internal pressure inside the battery casing increases as hereinabove described, the same safety valve works as a flow check valve to substantially avoid or minimize any possible entry of oxygen-rich gas into the battery casing. This is because, so long as the sealed lead storage battery is allowed to stand for a substantial time, and unless the communication between the atmosphere and the interior of the battery casing is interrupted, oxidization of the negative electrode plates tending to absorb oxygen tends to be accelerated, accompanied by the accelerated self-discharge of the lead storage battery.
Because of the employment of the dual-function safety valve, not only is the internal pressure inside the battery casing maintained at a safe value, but also during the normal use of the storage battery the negative electrode plates absorb oxygen, wherefore decompression is created inside the battery casing at all times, thereby ensuring the safe use of the sealed lead storage battery.
According to the prior art, the dual-function safety valve comprises a rubber cap mounted on the battery casing so as to close a tubular breathing passage which communicates with respective cell chambers inside the battery casing. The rubber cap is retained in position by a lid or cover member exteriorly mounted thereon so that the rubber cap will not separate away from the tubular breathing passages during the operation of the safety valve.
With the safety valve so constructed, the prior art sealed lead storage battery requires the use of the tubular breathing passage for the installation of the rubber cap. Also, depending on the type of material used to make the rubber cap, and the thickness, the hardness and the size of the rubber cap, the performance of the safety valve tends to vary. Moreover, a relatively large volume is required to accommodate the safety valve, and therefore, minimization of the size of the lead storage battery as a whole is often hampered.